St. Cewydd is said to have been one
of the many saintly sons of Caw of Prydyn, a Pictish king in the
Strathclyde area of modern Scotland. With the rest of his family, he would
have moved south to Edeirnion in Wales, around the early 6th century.
However, the evidence for this family relationship is mostly based on the
unreliable Iolo MSS and must therefore be treated as highly suspect.

Traditionally, Cewydd became a monk in St.
Cadog's monastery at Llancarfan (Morgannwg) and places in South
Wales named after him may date from this time spent in the area.
Llangewydd, near Bridgend, has lost its original church dedicated to him,
now only traceable in the fieldname, Caer Hen Eglwys. Lancaut near
Chepstow is also probably named for Cewydd; along with Cusop, near
Hay-on-Wye, and the extinct Capel Cawey in Monachlog Ddu (Pembrokes).
Perhaps he also took evangelical trips to Somerset, where Kewstoke is
believed to derive its name from Cewydd.

Elsewhere, place-name evidence shows that Cewydd eventually settled in
Elfael (Radnorshire) where he made a number of foundations. The churches
of Aberedw and Disserth are both dedicated to him. Cewydd's Retreat, Cil
Cewydd, appears in the adjoining parish of Llanfihangel Bryn Pabuan
and his hill-slope, Rhiw Gewydd, is a mountain track above Llandilo
Graban, possibly leading to his brother Meilig's home in Llowes. He
probably died in this region on a date variously said to have been on 1st,
2nd or 15th July. The latter appears to have been the most widely
accepted.

All three dates also have close associations with Cewydd's English
equivalent, St. Swithun.
Both were the 'Rain-Saints' of their respective nations and it seems
likely that these particular days were originally pagan Celtic festivals,
of some kind, related to the weather. It is popularly said that if it
rains on St. Cewydd's day, it will rain for forty days and forty nights!